1. Field
This disclosure relates to the enzymatic preparation of galactooligosaccharide (GOS) from lactose. More particularly, this disclosure relates to the sequential use of two different microbial lactase enzymes to maximize the degree of transgalactosylation during the digestion of lactose.
2. Description of Related Art
Galactooligosaccharides (GOS) are non-digestible carbohydrates that serve as the building block of oligosaccharides in human milk. GOS modulate the growth and activity of gastrointestinal microorganisms, and are therefore believed to promote a healthy balance of microorganisms in the gut. Among other things, GOS are believed to reduce levels of blood serum cholesterol, improve mineral absorption, and prevent colon cancer development. The properties of GOS depend significantly on the chemical composition, structure, and degree of polymerization (DP).
GOS can be formed by the digestion of lactose with β-D-galactoside galactohydrolases. β-D-galactoside galactohydrolases catalyze the hydrolysis of the galactosyl moiety from the non-reducing end of lactose. In addition, β-D-galactoside galactohydrolases can catalyze transgalactosylation in which a galactosyl moiety is transferred to a nucleophilic acceptor other than water, i.e. potentially any sugar present in a reaction medium. Transgalactosylation is a kinetically controlled reaction, and represents competition between the reactions of hydrolysis and synthesis. The ability to favor synthesis over hydrolysis depends on several factors, including the origin of the β-D-galactoside galactohydrolase and the initial composition of acceptor sugars in the medium (e.g. lactose and galactose) with which the enzymes are presented. If lactose is the initial substrate, transgalactosylation results in the production of GOS comprising a mixture of di-(DP2), tri-(DP3), and even higher oligosaccharides (DP4+) with or without a terminal glucose. The chemical structure and composition of a GOS (e.g. the number of hexose moieties and the types of linkages) affects its properties, such as the fermentation pattern by probiotic bacteria in the gut. The chemical compositions, structure, degree of polymerization, and yield of GOS also depends on the origin of the β-D-galactoside galactohydrolases utilized.
Many adults are lactose intolerant, and thus it is desirable to hydrolyze as much lactose as possible during the preparation of GOS from lactose. However, reaction conditions that favor the enzymatic digestion of lactose to, for example, less than 20% of the initial lactose concentration tend to also favor the digestion of GOS that is synthesized. Accordingly, reducing lactose concentration may result in reduced yield of GOS.